Commercial solid oxide fuel cells (SOFC) require both high performance and high mechanical integrity at a reasonable cost. Conventional cells for SOFC stacks have sought to achieve high performance by reducing the thickness of the electrolyte layer. As the electrolyte thickness has been reduced to less than ˜150 microns, it has become necessary for manufacturers to support the electrolyte layer with a mechanical support layer. Designs are being pursued that rely on the anode, cathode, and in some cases an inert layer, to serve as the load bearing member of the cell. While such approaches have been successful from an electrochemical perspective, the resultant mechanical properties of anode and cathode supported cells have required significant design compromises that limit overall performance of the systems. Enhancement of the mechanical strength of thin electrolyte layers to maintain an electrolyte-supported cell would provide significant advantages in cell stability and stack sealing.
Commercial SOFCs also requires high-quality, high-reproducibility seals. The critical parameters necessary for high-quality, high-reproducibility sealing are (1) that the cells be reproducibly flat, and (2) that the edges of the cells be dense. Electrolyte-supported cells, in which the mechanical support for the cell is a solid electrolyte, are more suited for simple sealing arrangements, because the porous electrode layer does not extend to the edge of the cell.
The difficulty of sealing SOFC stacks drives system designers toward larger planar cell areas. The larger the cell area, the fewer the number of repeat parts, such as interconnect plates, seals and other auxiliary components. These materials contribute significantly to stack weight, cost, thermal management, and reliability. Planar SOFC cells generally offer better configurations for high gravimetric power density stacks than tubular cells, provided that the sealing and mechanical integrity of the cells is sufficient for the application. The manufacture of large area SOFC components is difficult: the larger the area of the cell, the more likely that a performance-limiting flaw such as a crack, pinhole or other small defect will compromise the entire cell. For anode-supported cells, the primary challenges are maintaining flatness and electrolyte perfection over these large areas. Anode and cathode supported cells face serious issues during processing because of the sintering mismatch between the two electrode support layer and the dense electrolyte layer.
The difficulties associated with cell size and cell architecture relate to yet another potential barrier, the mechanical strength of the fuel cell in portable power supplies or vehicles. The support and the electrolyte layer must be mechanically stable for long periods of time at high temperature. The formation of defects or cracks in the electrolyte will critically affect the performance of a single cell and even the stack. The strength of the support and electrolyte layer becomes especially important when the cell area is increased, making the probability of critical flaws greater. Anode-supported cells based on various nickel-based cermet compositions inherently fail due to residual anode/electrolyte stresses developed during processing or stresses developed during redox processes. Overall, planar anode-supported cell designs show low potential as a stable platform for prolonged high temperature operations. Electrolyte-supported designs have a much greater potential for high temperature applications, since in this case, the support is composed of a singular composition which possesses respectable mechanical properties and is stable in both oxidizing and reducing atmospheres.
The use of SOFCs for applications such as transportation auxiliary power units requires extremely high levels of reliability. For planar stack geometries, which offer the highest gravimetric and volumetric efficiencies, the mechanical strength of the support structure of the cell is of paramount importance. To increase the strength, metal supports must be used or the relative strength of the ceramic support must be increased.